Synchronous motor starter



Oct. 27, 1942. E. H. STIVENDER SYNCHRONOUS MOTOR STARTER Filed April 29, 1939 2 Sheets-Sheet l Oct. 27, 1942.

A SYNCHRONOUS SPEED Filed April 29, 19 59 2 Sheets-Sheet 2 AVERAGE VALUE AVERAGE VALUE J l I 20 so so so I00 ZPOWERFACTOR 200 400 600 300 I000 I200 I400 woo woo STATOR AMPERES larly where the motor Patented Oct. 27, 1942 S PATENT OFFICE 2, 99,771 SYNCHBONOUS MOTOR STARTER Edward H. Stivender,

to Allis-Chalmers Mann! Milwaukee, Wia, a corporation of Delaware Application April 29, 1939, Serial No. 270,859 8 Claims. (Ci. 172-289) 11115 invention relates to the control of electrical apparatus in response to characteristics of alternating current utilized, and the invention is of particular utility in the control of synchronous alternating current motors.

It has been customary practice heretofore to start a synchronous motor by placing the same in operation initially as an induction motor, the direct current field member being provided with a special closed circuit winding in which alternating current is induced during the starting operation. And the speed of the motor, during such operation as an induction motor, will build up and approach, but not reach, synchronous speed. Hence, it has been customary to connect the direct current field winding, either manually or automatically, to a source of direct current supply when the speed of the motor, during the pose of pulling the motor locking it there for normal operation at synchronous speed.

Synchronous motors may be started with the circuit of the direct current field winding open, at either one point or at several points, the latter condition providing for reduced voltage at the open circuit terminals of the field winding, or with the circuit of such field winding closed, usually through a resistance, being wholly or substantially removed from the circuit on connecting the winding to the source of direct current, for pulling themotor into synchronism.

Prior practice in the matter of determining the proper instant for connecting, or effecting the connection of, the field winding to the source of direct current supply includes the use of devices responsive to the speed of the motor. Another prior method of efiecting the synchronization of synchronous motors, of advantage more particuis started under constant load, includes the utilization of the inherent reduction in the average current in the circuit of alternating current winding of the motor as the speed increases toward synchronizing speed, to automatically eilect the connection of the field winding to the direct current source at the desired sub-synchronous speed. And still other methods include the utilization of characteristics of the current, such as average amperage or voltage or frequency, induced in the field winding when the motor is started with the field winding closed, to automatically efiect connection or the field winding to the direct-current source.

and the resistance power factor It is known that, during the starting 0! definite pole synchronous motors of the ordinary commercial types which are started as induction motors having a closed circuit secondary winding, there is a wide variation in the value of the eifective current in the circuit of the alternating current winding of the motor, this current initially being. in some cases, approximately seven or eight times as great as full load current under normal operation, and decreasing to approximately to of the initial value as the motor closely reaches 90% to 05% of synchronous speed. And it is also known that, during the starting operation, there is considerable variation in the average power factor of the circuit of the alternating current winding of the motor, the being a minimum at the moment of connecting the alternatin current winding to the and increasing as the speed rises towards synchronous speed. this average Power factor varying from a value of 20% to 50% at thebeginningofthestartingperiod,ioavalue of 30% to at approximately 90% to 95% closed on itself during of synchronous speed, the initial sub-synchronom power factor being subject to considerable variation dependent uponthe load under which the motor is started. And. aside from this variation in average power factor during the starting period of the synchronous motor, there is a continuous cyclic variation in the instantaneous value of the power factor of the alternating current circuit, the periodicity of this cyclic variation varying inversely as the slip of the motor, and the amplitude of the cycles increasing to a very substantialextentasthemotorspeedrisesaboveavalue p xlmlting to of synchronous value. This cyclic variation in instantaneous values of power factor of the alternating current circuit of the motor is incident to the passing of the definite polar paths through the field element into and out of alinement with the rotating magnetic field established by the primary or alternating current winding of the motor, and, to a considerable extent, independently of whether the circuit of the direct current field winding is open or the starting p riod.

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Au adimtahle resistance M may he inclufied in the efmuif; e the E-s indicaiefi 5116 dra ings. the cenfiaef; finger 2mm gm. C. source 8., and whm the contact $2 and a namfnductive resistance 3% and a vafiable reactance dewce 3%, to the negative side 0f the D. C. central By vaflatian of the inductive eflect of the reactance device 39., as fifimugh adjustment of a. variable resistance it in a closed cimuif: including a windmg inductiveiy relafieei m The the time cantype rheasizzf; 28 hawof the electromagnet II, in applying direct current to the motor field and interrupting the short circuit through the'resistance I I to occur at such suitable interval after closure of contacts I, I I, as corresponds to. a favorable point in the slip cycle and facilitates pulling of the motor into synchronism. I

Operation of the electromagnet [2 causes closing of contactors 4i and 4| along with the switch 9 of the field circuit. Contactor 48. when in closed position, serves to complete an independent energizing circuit for the electromagnet II,

from the positive sideoi the D. C. control circuit, through the winding of the electromagnet l2, reactance l! and resistance M, to: the negative side of the D. C. control-circuit. Contactor 45 closes the circuit of the energizing winding of an electromagnet 46 which serves, when energized, to open the contactor l1 and thus the initial circuit through the contact I] and the energizing winding of the electromagnet l2. Energization of the electromagnet 46 may also serve to open a normally closed contactor 41 to remove a normal short circuit about the resistance 48 in the circuit of the energizing winding of the electromagnet 40. The opening movement 7 of contactor 41, on energization of the electromagnet 44, is preferably delayed to a slight extent by a conventional time relay device, as indicated. And energization of the electromagnet 48 also serves to cause closure, with -a suitably delayed action, of contactor II located in a circuit from the contact I! of the relay device, this auxiliary circuit by-passing or short circuiting the energizing winding of the electromagnet l2, and hence causing deenergization of such electromagnet. By reason of the time delay in the closure of the contactor BI and the-fact that the contact it of the relay device is normally out of engagement with the contact ll. afterthe 'cnotor is synchronized and thereafter engages the contact -l'l only in case the motor .pulls out of synchronism, the by-pass circuit, through the contactor II, is effective to short circuit the winding of the electromagnet1l only on the occurrence of abnormal running conditions.

Actuation oi the electromagnet I! also causes ll of an overload relay biased to closed position, the energizing winding of this relay being included in series in the circuit of the current transformer 25.

An auxiliary contactor I, biased to closed position, may also be included in the circuit of the winding of the under-voltage relay magnet 51, this contactor 4! being actuated under certain conditions by energization of a time delay electromagnet 62 whose energizing winding is completed by a contactor I, biased to closed position. The energizing winding of the contactor it is "included in series in the circuit of the field winding of the motor, and it is effective to open the contactor 83 on connection of thefield winding to the D. -C. source and to hold the contactor open only while the current in such winding remains above a predetermined minimum value. An auxiliary contactor l, biased to closed position, is actuated to open position by electromag- 'net 62 in opening the contactor Bl, opening of the contactor 85 serving to insert a normally short circuited resistance 61 in the circuit of the energizing winding of the electromagnet 42.

The supply of D. C. energy from the source 8 to the D. C. circuits, particularly the D. C.,control circuits, is controlled by a conventional form of contactor or switch which is closed and opened when the A. C. line switch 8 is closed and opened,

respectively. Such an auxiliary switch may be actuated separately from the A. C. line switch or .circuit breaker, but,the operation of such auxiliary switch is preferably-coordinated or interlocked, electrically or mechanically, with the operation of such line switch, so as to secure operation'of the auxiliary switch to closed and opened positions simultaneously with the line opening ofa normally closed contactor It, to

remove a normal short circuit about a resistance ll in the circuit of the winding I.l -:-ofthe electromagnet l4 of the relay device ",thercby reducing the tendency'ot the relay device toward such operation as causes engagement of the contact ilnger- II with fixed contact l1, after the electromagnet." has closed the D. C. ileld switch l to eflect synchronizingof the motor, ,and the latter might operate at leading power factor.

The circuit breaker I, with the auxiliary contactor ll completing the secondary circuit of the potential transformer 24, when in closedposition, is held latched therein in a conventional manner, and a tripping electromagnet-l4 is provided for tripping-the circuit breaker i and'the switch ll to open position. The: energizing .winding of this tripping magnet may be connected.

across the secondary oi' the potential transformer 28. or, as shown, it may be supplied withA. C. line current, the circuit of this energizing winding being closed by a contactor 5|, biased to open position, and held closed by an undervoltage relay or magnet 51 whose energizingwinding is in a circuit across the terminals of-the secondary-of the potential transformer 2', beyond the contactor It. The circuit of the undervoltage magnet preferably includes therein a contactor switch. The desired result may be attained by a switch between the D. C. source of supplyand the D. C. control circuit Ia; and such a controilingswitch of conventional form, indicated at 4!, may be separately closed substantially at the same time as the line switch 6 and opened substantially simultaneously with or subsequent to opening of the latter switch. However, as a conventional preferred operating expedient, the auxiliary controlling switch 8! is indicated as being mechanically interlocked with or secured the line switch 0 or its operating means, to

insure closure and opening of the contactor I and the line switch 8 substantially simultanecusly. ration of the relay device It toeflect desired connection of the'fleld winding 1 of the motor to the D. C. supply and interruption of theshort circuit through the field resistance II is dependent on the fact that, in the building up of the speed of the motor during the starting operation,'the current in the circuit of the A. C. winding of themotor decreases gradually and its average power factor, in addition to undergoinga substantial increase in value, also undergoes a cyclic variation, the frequency of this cyclic variation being twice that of the slip and varying with the frequency of the slip. These cyclic variations are of considerable amplitude when the speed of the motor rises above approximately of synchronous speed and continue to increase in amplitude, while decreasing in fre-' quency, as the speed closely approaches synchronism, and drop to zero as synchronous speed is reached. This cyclic variation in power factor of the circuit of the A. C. winding occurs independently of whether the circuit of theheld winding is closed or open, and it is occasioned of increasing duration during which the instantaneous power factor is below and above the average power factor. This characteristic is utilized in securing desired operation of the relay device l3, with its energizing windings 28, 23

' subjected to variations in the current and the varying frequency and magnitude of changes in the power factor from average value, to thereby efi'ect the connection of the field winding to the source ofdirect current at a predetermined motor speed, thus causing the motorto pull into synchronism.

In Fig. 2, curve Y represents approximate variations in the power factor, and curve Z approximate variations in the kilovolt amperes, of the circuit of the stator winding of a conventional synchronous motor of the rotating field type. Curve W, roughly an average of curves Y and Z, represents the resultant effect of varying kva. and power factor and, in its relation to the line X, the efiective force acting on the relay 5; during the starting operation of the motor, to hold the contact it away. from or move it into engagement with contact it. This line X represents the selected balance point of the relay device it}; and when the curve W is to' the right of the line X, the contact it is held withdrawn from the contact ii, and when the curve W. is to the left oi the line X, the contact it is moved into engagement with the contact ii. By suitable setting oi the adjustable rheostat 28, thebalance point of the relay, represented by the line X, may be correspondingly adjusted to secure desired efiective vibration of the relay at a point corresponding to a selected high sub-synchronous speed. With the curve W, as it is through= out its greater part, to the right of the line X, the energizing efiects oi the diflerential relay are such asto hold the contact finger it of the relay in engagement with contact or stop it.

Until the speed of the motor has risen to ap-' proximately 80% of synchronous speed, the frequency and magnitude of the cyclic variations in instantaneous power factor have little eflect in varying the mwer factor component of the efiective energizatlon of the cores it and of the relay device it. As the motor speed rises above approximately,80% of synchronous speed, and hence the slip decreases accordingly, the variations in instantaneous power factor become more pronounced, as indicated by the curve Y; and these variations in-the power factor produce a correspondingly varying value of efiective energization oi the cores id, id of the relay device, as indicated by the zigzag upper portion of curve portion W. As the value W drops to points to the left of the line X, the contact finger it vibrates in the space between fixed contacts 88 and ii; and at a motor speed corresponding to approximately 90% of synchronous speed, the power factor component of efiective energization of the cores it, it fluctuates in a decided manner at a graduaih decreasm irequency depending cally with contact ll, engagement with the latter causing closure of the circuit of the winding of electromagnet I2. The time interval of closure of contact finger IS with contact I'i becomes increasingly greater as the slip decreases, until, at a point corresponding to approximately of synchronous speed, this period of engagement with contact I! is of sufficient duration to exceed the time constant of the winding of circuit of the electromagnet II, this time constant being adjustably predetermined by the rhedstat 4| to insure application of direct current to field at a desired sub-synchronous value; and at such point, the electromagneti! is actuated to cause the field winding 1 to be connected with the D. C. source 8 and the short circuit through the resistance II to be removed.

With the hereinabove explanation of the oper-- atlon of the relay device i3, in effecting the application of direct current to the field winding, the function of the control system disclosed in Fig. l, in the control of a synchronous motor, will be more readily apparent. On closure of the circuit breaker 6, and incidental closure of the contactors 35 and 69, the stator winding of the motor is connected to the A. C. line; and the secondary circuit of the potential transformer 26 is closed by auxiliary switch, and the D. C. control circuit 8a is connected to the D. C. source 8 by the contactor 69; and. the motor starts as an induction motor with its field circuit closed through resistance H or, if desired, open at one or more points. Closureof the circuit of the potential transformer 26 completes the circuit through normally closed contacts 58 and BI to energize the low voltage magnet Eli, causing closure of the contactor '56 to thereby establish cuit corresponding to the upper zigzag portions of the curve W of Fig. 2; and, as the motor speed continues to rise, contact finger it periodically engages with contact ill and establishes an ener gizing circuit through normally closed contactor iii, energizing winding of electromagnet it, time delay reactancets, resistance 38, to the negative side of the direct current supply line. As described hereinabove, when thespeed of the motor has 'risen to approximately 95% of synchronous speed; the electromagnet i2 actuates the switch to connect the field winding 7! to the direct current supply line to pull the motor into synchronism and interrupt the short circuit through the resistance ll. v

With field switch 9 closed, the motor is thereupon pulled into synchronism, and the power factor of the A. C. circuit then follows the line Y to a value above 50%, corresponding to the value of the field current. While the motor remains in synchronism and normal field current is present, the power factor will not return to a 50% lagging value. And with the motor in synchronous operation, and the power factor of the A. C. circuit at any normal value for this operating condition, the efiective energization of the relay it is such as to withdraw the contact finger gizing winding 2| of the relay device l3.

it from, and hold it out of, engagement with the without the D. C. field being applied thereto, the

power factor will drop to a very low point and remain there, thus leaving contact I! closed until the D. C. field is applied, and the contactor 63 consequently opened, assuming that the latter occurs within a time interval, predetermined by the setting of the time delay associated with the electromagnet 62, after closure of the line switch 6 and contactor 69, as referred to hereinafter.

Actuation of the electromagnet ii to close the field switch 9 causes closure of the normally open contactors 43 and 45 and opening of the normally closed contactor 52. Closure of contactor 43 establishes a-holding circuit for the winding of electromagnet I! from the positive D. C. line, and independent of thecontactor 31 and relay contacts [6, H. As described hereinabove, opening of contactor 52 removes a normal short circuit about resistance 53in the circuit of ener- And closure of contactor 45 establishes the energizing circuit, through resistance 48, of the winding of the electromagnet 45 which is effective to open contactor 31 immediately and to close contactor 5| and open contactor 41 after a predetermined time interval. Opening, of the time delay contactor 41 removes a normal short circuit about resistance 48 in thecircuit of the winding of electromagnet 46. With the contactor 5| closed, subsequent engagement 01' contacts l6 and ll of the relay device, as will be referred to hereinafter, establishes a short circuit about the ener- V gizlng winding of the electromagnet l2.

Connection of the field winding 1 to the D. C. circuit 8 completes the circuit of the energizing winding of contactor 63, with consequent open ing of the latter and the energizing circuit of time delay electromagnet 82, the latter being adjusted to cause opening of normally closed contactor il in the line switch control circuit, including the energizing winding of the low voltage release magnet 51 and normally closed overload responsive contactor 58, a definite time after the line circuit breaker and auxiliary switches 35 and 69 are closed, if the D. C. field current has not been applied to the motor within such definite time, this tripping of the line circuit breaker being accomplished by opening of contactor 65.

This opening of the line switch 6, through opening of contactor 6|, may also occur in case the current in the D. C. field of the motor, when once applied, should drop to such predetermined value as permits the contactor 83 to drop to closed position and remain there for a predetermined time corresponding to the time delay period of the electromagnet S2.

The actuation of the time delay electromagnet 62 also effects opening of the normal closed contactor 65, to thereby insert a normally short circuited resistance 61 in' the circuit of the energizing winding of the electromagnet 62.

The low voltage responsive contactor 51 and the overload responsive contactor 58 are effective at any time, on the occurrence of predetermined conditions in their actuating windings, to cause tripping of the line switch 5 to open position.

with'the motor placed in synchronous opera tion as hereinabove described, it will continue in synchronous operation under normal conditions. However, the apparatus disclosed herein is operative, in the event that the motor should fall out of step or synchronism due 'to overload or other causes, to remove the D. C. field of the motor, thus permitting it to operate temporarily as an induction motor, and to thereafter automatically resynchronize the motor, if later operating conditions permit the same within a predetermined time after the motor has fallen out of step. If the motor should fall out of synchronous step, the phase angle between current and voltage increases and the power decreases and will reverse as the motor falls one hundred and eighty electrical degrees out of step. As the motor falls out of step, the power factor, instead of following the definite line of average power factor, indicated in Fig. 2, assumes varying instantaneous values such as were present during the latter stages of placing the motor in synchronous operation, and if this out-of-step operation ofv the motor reaches a degree such that the instantaneous value of the power factor, and hence the resulting effective energization of the electromagnet's l4, l5 of, the relay device is such that electromagnet l4 predominates, corresponding to a value of'W to the left of the line X of Fig. 2, then the contact finger l6 engages contact II to immediately establish a lay-pass circuit, through closed contactor 5|, which short circuits the energizing winding of the electromagnet l2, with consequent deenergization of the latter and opening of the switch 9, to effect disconnection of the field winding of the motor from the direct current source.

Deenergization of the electromagnet l2 reestablishes the control circuits which, as originally described hereinabove, were effective, with the circuit breaker 6 and auxiliary switches 35 and 6! closed, to accomplish the automatic synchronization of the motor; and the motor, under these conditions, tends to operate as an induction motor, at a speed depending upon the load on the motor. Ifthe abnormal load orother condition, which caused desynchronization' to occur, persists fora period corresponding to the time delay of electromagnet 62, the circuit of which is closed at this time through contactor '63, then electromagnet 82 opens contactor 6|, thus opening the circuit of the windingof the low voltage magnet 51, with consequent opening of contactor 56 and the circuit breaker 6.

However, if the disturbance which caused desynchronization of the motor is of short duration after opening of the field switch 9, and the load on the motor will permit resynchronization in the normal manner, relay device I3 will again function in its normal manner, described hereinabove, to reestablish the D. C. field of the motor before the end of the time delay period in the opening of the contactor Si by the electromagnet 6i, and bring the motor into synchronous op- 76 to, it will be apparent that the invention is ap.

plicable to starting the motor by initially applying a reduced voltage, through an auto-transformer or similar device, and thereafter, when the motor has reached a predetermined percentage Of synchronous speed, applying full voltage to the motor. And the change from starting to full voltage connections of the motor to the A. C. supply line may be in accordance with any known methods, i. e., responsive tospeed of the motor or a definite time interval after establishing start- 10 ing connections; and a relay device of the character of the hereinabove described relay device i3, but preferably calibrated to have a balance point, corresponding to the line X of Fig. 2, and

point of initial engagement of the contacts, is 1 5 lay device to establish a second range of operation at higher speeds, the same relay device may be of increased range of utility in accomplishing the double function of changing over from startin to running connections and synchronizing the motor.

It will be apparent that, in accordance with the present invention, a single type or size of controlling relay device may, with suitable inherent adjustment ofthe parts thereof, be eifectively and efficiently used in controlling synchronous motors of a wide variety of sizes, for synchronizing the-motor at a most favorable point for each particular motor and for pull-out protection therefor.

Although the present invention has been illustrated and described in connection with certain embodiments, it will be apparent that various m changes and modifications may be made therein without departing from the spirit of the invention disclosed or defined'by the appended claims.

It is claimed and desired to secure by Letters Patent:

d5 1. Controlling apparatus for a synchronous motor having a winding connectable to an alternating current supply circuit and a field winding connectable to a source of direct current, comprising an electromagnetic device including an armature element and energizing windings therefor whose resultant energizing effect is responsive to electrical conditions dependent upon the rate and magnitude of cyclic variations in the power factor of said alternating current circuit when the latter is connected to said motor, a movable contact, a second contact, said movable contact being operable by said armature element into and out of engagement with said second contact in synchronism with cyclic variations of the power factor of said alternating current circuit to. a predetermined degree from an average value, a switch for connecting the field winding of said motor to the direct current source to establish synchronous operation of said motor, and means rendered operable through said contacts for actuating said switch to closed position only upon said contacts remaining in engagement for a predetermined time interval corresponding to operation of said motor at a predetermined sub-syn- 7o chronous speed.

2. Controlling apparatus for a synchronous motor having a winding connectable to an alter nating current supply circuit and a field winding connectlble to a source of direct current, comprising an electromagnetic device including ar armature element and energizing windings therefor whose resultant energizing effect is responsive to electrical conditions dependent upon the rate .and magnitude of cyclic variations in the power factor of said alternating current circuit when the latter is connected to said motor, a movable contact, a second contact, said movable contact being operable by said armature element into and outof engagement with said second contact on variation of the power factor of said alternating current circuit to a predetermined degree from an average value, and means rendered operable by "said movable contact when in engagement with said second contact for effectively connecting the field winding of said motor to the direct current source to establish synchronous operation of the motor, said means including a device for delaying the connection of said field winding tothe direct current source until said contacts are in engagement for a time interval corresponding to a predetermined sub-synchronous speed of operation of said motor. v

3. Controlling apparatus for a synchronous motor having a winding connectable to an alter-,

nating current supply circuit and a. field winding connectable to a source of direct current, comprising an electromagnetic device including an armature element and energizing windings therefor responsive to instantaneous current and voltage conditions dependent upon power factor conditions of said alternating current circuit when the latter is connected to said motor, a movable contact, a second contact, said movable contact being operable by aid armature. element into and out of engagement witn'said second contact on cyclic variation of the power factor of said alternating current circuit to a predetermined degree from an average value, and means rendered operable by said movable contact when in engagement with said second contact for efl'ectively connecting the field winding of said motor to the direct current source to establish synchronous operation of the motor, said means including a device for delaying the connection or said'field winding to the direct current source until said contacts are in engagement for a time interval corresponding to a predetermined sub-synchronous speed of operation of said motor, said armature element being efiective to withdraw said movable contact from engagement with said second contact on attainment of synchronous operation by said motor and to again move said mov-. able contact into engagement with said second contact on said motor falling out of synchronism to cause said means to disconnect the field winding of said motor from the direct current source.

a Controlling apparatus for a synchronous motor having a windingconnectable to an alternating current supply circuit and a field winding connectable to a source of direct current, com

prising an electromagnetic device including an armature element and ener izing windings therefor responsive to instantaneous current and voltage conditions dependent upon power factor conditions of said alternating current circuit when the latter is connected to said motor, a movable contact ,element, a second contact element, said 7 movable contact element-being operable by said armature element into and out of engagement with said second contact on cyclic variation of the power factor of said alternating current circuit to a predetermineddegree from an average value, instrumentalities associated with said electromagnetic device for varying the predetermined point of initial closure of said contacts with respect to the speed of said motor, and means rendered operable by said movable contact element when in engagement with said second contact for effectively connecting the field winding of said machine to the direct current source to establish synchronous operation of the motor, said means including a device for delaying the connection of said field winding to the direct current source until said contacts are in engagement for a time in terval corresponding to a predetermined sub-synchronous speed of operation of said-motor, said armature element being effective to withdraw said movable contact from engagement with said second contact on attainment of synchronous operation by said motor and to again move said movable contact into engagement with said second contact on said motor falling out of synchronous step to cause said means to disconnect the field winding of said'r otor from the direct current source.

5. Controlling apparatus for a synchronous motor having a winding connectable to an alternating current supply circuit and a field winding connectable to a source of direct current, comprising an electromagnetic device including an armature element and energizing windings therefor responsive to electrical conditions dependent upon cyclic variations of the power factor of said alternating current circuit to a predetermined degree from an average value when said motor is connected to said circuit, instrumentalities associated with said electromagnetic device for varying the degree of responsiveness of said device, a

circuit controlling element operable by said ar-' mature element and freely movable thereby into and out of effective circuit controlling position in synchronism with said cyclic variations of the power factor, and means rendered operable by said circuit controlling element when the latter is held in effective circuit controlling position for a predetermined time interval for effectively connecting the field winding of said motor to the direct current source at a predetermined subsynchronous speed of'the motor to establish synchronous operation thereof and for disconnectin said field winding from the direct current source on the falling of said motor out of synchronous step.

6. Controlling apparatus for a synchronous motor having a winding connectable to an alternating current supplyv circuit and a field windin connectable to a source of direct current, comprising an electromagnetic device including an armature element and energizing windings therefor responsive to electrical conditions dependent upon the frequency and magnitude of cyclic vari-' ations of the power current circuit to a predetermined degree from an average value during operation of said motor in its upper range of sub-synchronous speeds, a circuit controlling element operable by said armature element and freely movable thereby into and out of effective circuit controlling position in synchronism with said cylic variations of the power factor, and means rendered operable by said circuit controlling element when the latter is held in effective circuit controlling position for a predetermined time interval for effectively connecting the field winding of said motor to the direct mined sub-synchronous factor of said alternating current source at a predetermined sub-synchronous speed of the motor to establish synchronous operation thereof and for disconnecting said motor from the direct current source on said machine falling out of synchronous step, and for thereafter reconnecting said field winding to the direct current source to resynchronize said motor in the event that conditions permitting such resynchronization exist within a predetermined time interval after said motor falls out of synchronous step.

'1. In combination, a synchronous motor, an alternating current supply circuit for supplying the armature winding of said motor, a direct current supply source for supplying the direct current field winding of said motor, and means for automatically establishing the connection of said field winding to said direct current source when said armature winding is supplied from said alternating current circuit and said motor reaches a predetermined sub-synchronous operating speed, said means comprising an electromagnetically actuated circuit-controlling device having a contact element freely vibrating into and out of circuit-closed position in response to instantaneous values of current and voltage and the phase relation therebetween present in said armature circuit when said motor is operating at sub-synchronous speeds approaching synchronism, and a time-delay device rendered operative on movement of said contact element to circuitclosed position, and said means effecting connection of said fleld' winding to said direct current circuit only after said contact. element has been in circuit-closed position for a predetermined time, determined by said time-delay device and corresponding to said predeterspeed. I 8. Controlling apparatus for a synchronous motor having an armature winding connectable ternating current circuit with the latter connected to said motor, a movable contact, and a second contact, said movable contact being operable by said armature element into and out of engagement with said second contact in response to said cyclic variations of a predetermined degree in the effective resultant energization of said electromagnetic device, and means rendered operable by said movable contact when in engagement with said second contact for effectively connecting the field winding of said motor with said direct current source to establish synchronous operation of said motor, said movable contact being automatically movable to inoperative position upon attainment of synchronous operation of. said motor and being thereafter movable into engagement with said second contact upon said motor falling out of synchronous operation to thereby ei!ect disconnection of said field winding from said direct current source.

EDWARD H. STIVENDER. 

